Systems and methods for measuring nitrate levels

ABSTRACT

The systems and methods described herein relate to the measurement of nitrate levels in a sample of gas, for example, air, exhaust, or other sources of gas. Moreover, the systems and methods described herein are capable of operating using short sample collection periods, permitting rapid data collection and finely time-resolved nitrate monitoring over a span of time. Additionally, ambient nitrate can effectively be distinguished from other airborne particles, such as sulfate and carbon.

This application is based on U.S. Provisional Application No. 60/158861,filed Oct. 12, 1999, the specification of which is hereby incorporatedby reference.

BACKGROUND OF THE INVENTION

Particulates are tiny clumps of soot, dirt, and various chemicals thathave been linked to a wide variety of health problems—asthma, and higherrates of disease affecting the cardiovascular system or lungs. Since1987, EPA standards have governed all particulates under 10 micrometersin diameter. This category of particulate matter is called PM10.Recently, however, studies have suggested that the most dangerousparticles are actually the smaller ones, which penetrate deeper in thelungs' aereoles. Thus, new regulations will build in a separate standardfor particles less than 2.5 micrometers in diameter—PM2.5.

While PM10 contains a lot of wind-blown soil, PM2.5 is derived mainlyfrom burning fossil fuels. PM2.5 typically contains a mixture ofelemental carbon, organic carbon, sulfate and nitrate particles,, andacid droplets. It is unlikely that all components of PM2.5 contributeequally to the observed health effects, yet the present lack ofsufficient data quantifying the individual components prevents the EPAfrom separately regulating these components. Because regulating PM2.5collectively is not a cost-effective solution, the agency is under greatscientific, industrial, and political pressure to specifically identifysources of the observed particle health-effects., Thus, interest inmeasuring the individual components of PM2.5 has increased dramaticallyover the last few years.

A number of methods are known for measuring atmospheric nitrate levels.Koutrakis et al., Environ. Sci. Technol. 22:1463, 1988 disclose anintegrated sampling method (Harvard/EPA Annular Denuder System (HEADS))which is designed to measure various atmospheric components includingparticulate nitrate. The method provides a non-quantitative conversionof particulate nitrate to nitric acid vapor by collection of atmosphericfine particles on a Teflon filter, with a sodium carbonate-coated filterdownstream to collect nitric acid vapor produced by volatization ofammonium nitrate and by the reaction of ammonium nitrate with acidicsulfate particles.

Wendt et al., “Continuous monitoring of atmospheric nitric oxide andnitrogen dioxide by chemiluminescence” in Methods of Air Sampling andAnalysis, editor, J. P. Lodge Jr., Lewis Publishers, Chelsea, Mich., pp415-421 (1989), disclose a continuous chemiluminescent NO_(x) detectionmethod. Yamamoto et al., Anal. Chem., 1994, 66, 362-367, describe anitrate analysis method relying on chemiluminescent NO, detection.NO_(x) generally refers to NO₂ and NO taken together.

Brauer et al., Environ. Sci. Technol. 24:1521, 1990 disclose a methodfor the continuous measurement of nitrous acid and nitric acid vaporswhich does not distinguish between the two species. Klockow et al.,Atmospheric Environment, 1989, 23, 1131-1138, disclose thermodenudersystems for the discontinuous measurement of nitric acid vapor andammonium nitrate. Buhr et al., Atmospheric Environment, 1995, 29,2609-2624, teach a denuder for sampling nitric acid, nitrate, andsulfate. Wolfson et al., U.S. Pat. No. 5,854,077, present a continuousdifferential nitrate measurement method.

Many of these and other existing methods for nitrate measurement requirelabor-intensive, manual collection of 24-hour integrated samples andlaboratory analysis of the collected components. Not only are suchsamples expensive to collect, but the lengthy collection period preventsthe detection of cycles and patterns which occur over the course of aday. Convenient techniques which offer improved temporal resolution andare capable of unifying the collection and analysis processes are badlyneeded now to reveal these daily patterns, both for epidemiologicalresearch and for regulatory monitoring.

SUMMARY OF THE INVENTION

The systems and methods described herein relate to the measurement ofnitrate in gas samples by collection and analyzing samples by atechnique which permits a short cycling time. Thus, in one aspect, theinvention provides a system for measuring nitrate levels having a sampleinlet for receiving a sample of gas, a collection body coupled to saidsample inlet, a filter mounted within said body to collect particlesfrom said sample of gas, a heater coupled to the body to heat the body,a gas inlet coupled to said body to provide a flow of gas through saidbody, and a detector coupled to said body to measure an NO_(x)concentration.

In a certain embodiment, the system further comprises a source of gascoupled to said gas inlet. The gas may be nitrogen or another gas whichis substantially free of oxygen.

In another embodiment, the system also includes a catalyst, coupled tosaid body and to said detector, capable of reducing NO₂ to NO. Thecatalyst may comprise molybdenum, carbon, or ferrous sulfate.

In certain embodiments, the detector included in the system has. a lightsensor, and may further include an ozone generator, for example, for thedetection of the chemiluminescent oxidation of NO. In anotherembodiment, the detector includes an infrared sensor. In yet anotherembodiment, the detector includes a material which reversibly binds NO.

In one embodiment, the filter comprises quartz fibers.

In yet another embodiment, the system includes an extractor coupled tothe sample inlet and to the collection body to substantially remove NO₂from the gas sample. The extractor may comprise a hydroxyl-bearingsolvent and a base, e.g., glycerol and an organic base, e.g., an amine,such as triethanolamine.

In yet another embodiment, the system also includes a selectionplatform, situated between the sample inlet and the extractor, tosubstantially remove particles larger than about 2.5 microns. Theselection platform may be a filter, an inertial impactor, or any othersuitable device.

In one embodiment, the system further includes a cooling system to coolthe collection body.

In yet another aspect, the invention relates to a method for measuring alevel of nitrate by receiving a gas sample, collecting nitrate particlesfrom the gas sample on a filter, passing a stream of gas substantiallyfree of oxygen over the collected particles, volatilizing the collectedparticles by heating to generate NO_(x), and measuring a level ofNO_(x).

In one embodiment, the method further includes substantially removingNO₂ prior to collecting nitrate particles, e.g., by passing the receivedsample over a hydroxyl-bearing solvent and a base, e.g., an organic basesuch as triethanolamine.

In another embodiment, the method further includes removing particleslarger than about 2.5 microns from the received gas sample, e.g., bypassing the received sample through an inertial impactor or by passingthe received sample through a filter.

In one embodiment of the method, passing a stream of gas includespassing a stream of nitrogen over the collected particles.

In yet another embodiment, the method further comprises reducinggenerated NO₂ to NO using a metal catalyst, e.g., by contacting the NO₂with a molybdenum catalyst.

In certain embodiments, measuring a level of NO_(x) includes reacting NOwith ozone. In yet another embodiment, measuring a level of NO_(x)includes detecting infrared absorption. In certain other embodiments,measuring a level of NO_(x) includes adsorbing NO_(x) on a conductivematerial.

In one embodiment, collecting nitrate particles comprises collectingnitrate particles on a filter comprising quartz fibers.

In another embodiment, volatilizing the collected particles includesrapidly heating the collected particles to at least 300° C.

In yet another aspect, the invention provides a system for measuringnitrate levels, including a sample inlet to receive a sample. of gas, anextractor coupled to said sample inlet to substantially remove NO₂ fromthe gas sample, a collection body coupled to said sample inlet, aninertial impactor mounted within said body to collect particles from thegas sample, a current source coupled to the inertial impactor to heatthe inertial impactor and generate NO_(x), and a detector coupled tosaid catalyst to measure an NO_(x) concentration.

In yet another aspect, the invention relates to a method for measuring alevel of nitrate by receiving a gas sample, substantially removing NO₂from the gas sample, collecting nitrate particles from the gas samplewith an inertial impactor, passing a stream of gas substantially free ofoxygen over the collected particles, volatilizing the collectedparticles by heating to generate NO_(x), and measuring a level of NO_(x)generated by the heated particles.

In yet another aspect, the invention provides a system for measuringnitrate levels having means for receiving a sample of gas, support meanscoupled to the means for receiving, means for collecting particlescoupled to the support means, means coupled to the support means, forheating the support means to generate NO_(x), means, coupled to thesupport means, for flowing a stream of gas through the support means,and means for measuring an NO_(x) concentration coupled to the supportmeans.

In one embodiment, such a system also includes means for substantiallyremoving NO₂ from the sample of gas, coupled to said means for receivingand said support means.

In another embodiment, such a system further includes means for reducingNO₂ to NO, coupled to the support means and to the means for measuring.

In yet another aspect, the invention relates to a method ofmanufacturing a nitrate measurement apparatus by providing a sampleinlet for receiving a sample of gas, coupling a collection body to thesample inlet, disposing a filter within the body, coupling a heater tothe body, coupling a gas inlet to the body, and coupling an NO_(x)detector to the body.

In one embodiment, the method further comprises disposing an NO₂extractor between said sample inlet and said collection body.

In another embodiment, the invention further comprises disposing acatalyst capable of reducing NO₂ to NO between said collection body andsaid NO_(x) detector.

BRIEF DESCRIPTION OF THE FIGURES

The following figures depict certain illustrative embodiments of theinvention in which like reference numerals refer to like elements. Thesedepicted embodiments are to be understood as illustrative of theinvention and not as limiting in any way.

FIG. 1 depicts a system for measuring nitrate levels as describedherein.

FIG. 2 illustrates the accuracy of a method for measuring nitrate levelsas described herein.

FIG. 3 shows the effect of atmospheric conditions on the methoddescribed herein.

FIG. 4 demonstrates a method of distinguishing between nitrate and NO₂in a sample of gas using the systems and methods described herein.

FIGS. 5A and B present nitrate measurement results obtained over a72-hour period.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

The description below pertains to several illustrative embodiments ofthe invention. Although many variations of the invention may beenvisioned by one skilled in the art, such variations and improvementsare intended to fall within the compass of this disclosure. Thus, thescope of the invention is not to be limited in any way by the disclosurebelow.

The systems and methods disclosed herein are useful for measuringnitrate levels, for example, in the atmosphere, and may be capable ofperforming sample collection and analysis within about ten minutes.Thus, variability of nitrate levels can be determined over relativelyshort intervals, e.g., for use in epidemiological studies, regulatorymonitoring, or other research. Furthermore, the system can be assembledor manufactured using convenient, commercially available components.

An exemplary system 100 for measuring nitrate levels is depicted in FIG.1. The system 100 includes a sample inlet 105, an extractor 110, acollection body 115, a filter 120, a heater 125, a cooling system 130, acatalyst 135, a detector 140, a gas inlet 145, and a gas source 150.Other components, such as a control system, a data acquisition andrecording system, or a second independent heater may optionally beincluded. Variations on the depicted system which are capable offunctioning as described herein will be apparent to those of ordinaryskill in the art and are intended to be encompassed by this disclosure.

A sample of gas, such as a sample of air or exhaust, may be received bythe system using sample inlet 105. The sample of gas may be forced intothe system 100, for example, by passing an exhaust stream through thesystem 100. Alternatively, the sample of gas may be drawn into thesystem 100 by a vacuum, e.g., by providing a vacuum beyond the detector140, or by utilizing the Bernoulli effect, e.g., by passing a stream ofgas rapidly past the inlet 105, e.g., using the gas inlet 145. Thesample inlet 105 may include a selection platform for removing particleslarger than about 2.5 microns, such as an inertial impactor or a filter,as is well known in the art. The sample of gas may then pass into theextractor 110 to remove contaminant gases. The extractor 110 may be adenuder, such as the honeycomb denuder described in U.S. Pat. No.5,302,191 or an annular denuder, another diffusion denuder, or any othersystem known in the art for removing gases from a sample of gas. Forexample, the extractor 110 may include an acidic material, such ascitric acid or sulfuric acid, to trap basic compounds, such as ammonia.In one embodiment, the extractor 110 is selected to remove at least 50%,or at least 90%, or even at least 95% of the gaseous NO₂ from the sampleof gas, as gaseous NO₂ may introduce error into the nitrate measurement.Such an extractor may include a hydroxyl-bearing solvent, such asethylene glycol, propylene glycol, glycerol, benzyl alcohol, or anotherhydroxylic solvent, and a base, including an inorganic base, such as ametal carbonate, bicarbonate, hydroxide, or phosphate, e.g., sodiumhydroxide or potassium carbonate, and/or an organic base, such as anamine, e.g., 1,8-bis(dimethylamino)-naphthalene, diazabicyclooctane,diazabicyclononane, triethanolamine, diethanolamine,N,N-dimethyl-2-hydroxymethylaniline, or another organic base. In certainembodiments, the hydroxyl-bearing solvent and the organic base areselected to have low vapor pressures at atmospheric pressure, e.g., lessthan 50 Torr, or less than 10 Torr. Other systems for removing NO₂ orother selected contaminants are known in the art, and may be used aloneor in any combination to remove any such compounds from the sample ofgas.

The sample of gas may then pass into the collection body 115 and throughfilter 120. The filter 120 may then trap nitrate particles, in additionto other particles of similar size, e.g., about 2.5 microns or less,while allowing gaseous compounds to pass through. The collection bodymay be composed of any material capable of withstanding sufficient heatto perform the method as described herein, such as metal, ceramic,glass, quartz, or other heat-resistant material. For example, thecollection body may be composed of steel, molybdenum, or an alloycomprising either material. The filter may be composed of any suitablematerial, e.g., quartz fibers, glass fibers, metal, or other materialcapable of withstanding temperatures sufficient to volatilize thetrapped particles. A stream of gas substantially free of oxygen, e.g.,including less than about 5% or less than about 1% oxygen, such asnitrogen gas, helium, or argon, may then be passed over the trappedparticles. This procedure helps to reduce unwanted oxidation of ammoniaor other low oxidation state nitrogen-containing compounds, such asammonium sulfate, during heating. The gas may be introduced using gasinlet 145 from gas source 150.

The heater 125 may then heat the filter 120. or the collection body 115to volatilize the trapped particles. The heater may perform thisfunction by any means known in the art. For example, the heater 125 maygenerate heat itself, such as with a heating element, e.g., a nichromewire or a heat lamp, used to heat the collection body or filter, or itmay apply current to the filter 120 or the collection body 115 to heatthat element by resistance, or it may heat the sample by any other meansknown in the art. In addition to heating the collection body 115 and/orthe filter 120, the heater 125 or a second heater may heat all or aportion of the path between the collection body 115 and the catalyst135. Upon volatilization, nitrate may be converted to species such asHNO₃, NO₂, and NO which are carried by the stream of gas to the catalyst135.

In certain embodiments, for example, wherein an extractor 110 is notused to remove NO₂ from the gas sample, a portion of the gaseous NO₂ inthe sample of gas may be adsorbed by material on the filter, such assoot or other particulate matter, rather than passing through thefilter. Upon heating such NO₂ may be desorbed at a temperature belowthat at which nitrate begins to substantially volatilize. In suchembodiments, it may be advantageous to heat the collection body 115 andfilter 120 gradually in order to release this unwanted NO₂ prior todetection and measurement of the NO_(x) species liberated byvolatilization of nitrate. NO_(x), as used herein, refers generally toNO and NO₂. By this method, more accurate nitrate determinations may bemeasured. Rapid heating, however, may permit more rapid cycling betweencollection and analysis phases. Similarly, cooling system 130 may coolthe collection body 115 and/or filter 120 by any means, such as bypassing an unheated fluid, e.g., air or water, over the exterior surfaceof the apparatus, to further enable more rapid cycling betweencollection and analysis phases. Thus, the speed of heating may beadjusted to balance cycling time with measurement accuracy, depending onthe needs of a particular situation and, the relative importance ofaccounting for NO₂ in the sample of gas.

The catalyst 135 may be any material, such as a molybdenum or carbonconverter, ferrous sulfate, or any other material capable of reducingNO₂ to NO, as is known in the art. In embodiments where a detector 140is used which is capable of simultaneously detecting NO₂ and NO, acatalyst 135 need not be included in the system, and the stream of gasmay flow directly from the collection body 115 and filter 120 to thedetector 140.

The detector 140 may be any component capable of detecting the amount ofNO_(x) in the stream of gas. A number of methods are known for detectingNO_(x) in flowing gas streams. Perhaps the most well known and widelyused process involves instruments using the chemiluminescent reaction ofnitric oxide (NO) and ozone. The process operates by the reaction ofozone and nitric oxide within a reaction chamber having a transmissivewindow, allowing light produced by the chemiluminescent reaction to-bemonitored by a detector. Typical components using this process may befound in U.S. Pat. No. 3,967,933 to Etess et al.; U.S. Pat. No.4,236,895 to Stahl; U.S. Pat. No. 4,257,777 to Dymond; U.S. Pat. No.4,315,753 to Bruckenstein et al.; U.S. Pat. No. 4,657,744 to Howard;U.S. Pat. No. 4,765,961 to Schiff; and U.S. Pat. No. 4,822,564 toHoward. The use of a chemiluminescent nitrogen oxide monitoring devicein controlling a nitrogen oxide removal unit placed on the outlet of aboiler is shown in U.S. Pat. No. 4,188,190 to Muraki et al. Becausethese systems are typically not capable of detecting NO₂ in the gasstream, a catalyst 135 may be employed in conjunction with such adetector.

Another procedure involves the use of an infrared beam, detector, and acomparator chamber. In U.S. Pat. No. 4,647,777 to Meyer, a beam ofinfrared light is passed through a gas sample and into a selectiveinfrared detector. The beam is split and one portion passes through achamber containing a fluid that absorbs the spectral wavelengths of theselected gas. The two beams are compared and the difference between thetwo beams gives an indication of the amount of a selected gas in thesample.

A semiconductor NO_(x) sensor is described in U.S. Pat. No. 5,863,503.The resistance of this sensor is altered by the absorption of NO andNO₂. Such a detector 140 may thus simultaneously measure NO and NO₂levels, and therefore may function accurately in the absence of acatalyst 135.

One of the above detectors, or any other detector capable of measuringNO or NO₂ concentrations, may be employed as detector 140. In the caseof a detector which is capable of detection NO₂ but not NO, it may beadvantageous to oxidize NO in the stream of gas to NO₂, for example,using an ozone generator or other source of oxidant. The detector 140may include or may be coupled to a processor, plotter, or otherrecording apparatus for displaying, recording, or storing data collectedby the detector 140.

In certain embodiments, the filter 120 may be replaced by an inertialimpactor, which is also known to be useful for collecting particulatematter from a stream of gas. In order to volatilize the collectedsample, the inertial impactor may be heated directly, or indirectly, asdescribed above for a filter embodiment. Otherwise, the system isanalogous to the system described above. Thus, in one embodiment, aninertial impactor is used in a system as described above which uses anNO₂ extractor, such as a diffusion denuder, as described above.

A system 100 as described above may be manufactured by coupling a sampleinlet to a collection body, disposing a filter in said collection body,coupling said collection body to an NO_(x) detector, and coupling a gasinlet to said body. In certain embodiments, the method may furtherinclude coupling an extractor, such as an NO₂ extractor as describedabove, between said sample inlet and said filter. When the detectoremployed does not adequately detect NO₂, a catalyst may be disposedbetween said detector and said collection body to reduce NO₂ to NO.Alternatively, if the detector employed does not adequately detect NO,an oxidizer may be disposed between said detector and said collectionbody. The components included in such a system may be any of thecomponents set forth above or components that function equivalently oranalogously.

The following examples are provided solely to further illustrate thenature and advantages of one embodiment of the present invention and arenot intended to limit the scope of the invention in any way.

Exemplification

A system as described above and depicted in FIG. 1 was tested todetermine the accuracy and utility of the measurements recorded thereby.

FIG. 2 shows that as nitrate concentration in the gas sample increases,instrument response increases in turn. Furthermore, the very linear fitindicates that the instrument provides a linear response and shouldmeasure nitrate levels accurately over a broad range of concentrations.

FIG. 3 shows that the introduction of species, such as water (relativehumidity (RH) saturated) or ammonia, into the sample of gas does notsignificantly affect the nitrate level readings of the instrument. Inall cases, the peak area is relatively similar.

FIG. 4 illustrates how NO₂ in the gas sample as collected and NO₂released from volatilization of nitrate particles can be distinguishedusing the present method, even in the absence of an extractor.

FIGS. 5A and 5B present data collected from atmospheric air samples overthree-day periods. Considerable variation can be seen within a given24-hour period, and these variations can be elucidated because of therelatively short collection-analysis cycles possible using the systemsand methods disclosed above.

All articles, patents, and other references set forth above are herebyincorporated by reference. While the invention has been disclosed inconnection with the embodiments shown and described in detail, variousequivalents, modifications, and improvements will be apparent to one ofordinary skill in the art from the above description. Such equivalents,modifications, and improvements are intended to be encompassed by thefollowing claims.

We claim:
 1. A system for measuring nitrate levels, comprising a sampleinlet for receiving a sample of gas, a collection body coupled to saidsample inlet, a filter mounted within said body to collect particlesfrom said sample of gas, a heater coupled to the body to heat the body,a gas inlet coupled to said body to provide a flow of gas through saidbody, a detector coupled to said body to measure an NO_(x)concentration, and an extractor coupled to said sample inlet and to saidcollection body to substantially remove NO₂ from the gas sample, whereinthe extractor comprises a hydroxyl-bearing solvent and a base, andfurther wherein the hydroxyl-bearing solvent is glycerol and the base isan organic base.
 2. The system of claim 1, wherein the organic base isan amine.